1. Field of the Invention
This invention relates generally to the wood processing industry, and more specifically relates to a method and apparatus for pretreating logs before conventional debarkers, or for replacing conventional debarkers in removing bark from logs.
While generally having utility for removing bark from all log species, the invention is seen as having its greatest advantage for removing bark from log species normally considered to be difficult or troublesome to debark.
2. Description of Prior Art
In many log utilizing industries, such as the papermaking industry, it is desirable to remove the bark from whole logs before further processing. In the past, several different types of debarkers have been used, generally groupable into three main categories based on the operating principles by which the bark is removed.
One general category of debarker, known as hydraulic debarkers, utilizes high pressure water jets to separate the bark from the log. Hydraulic debarkers include designs in which a single water jet traverses the log longitudinally, initially opening the bark layer. The log is rolled on its longitudinal axis, and the water jet used to separate the bark further around the circumference of the log, until the bark is completely separated from the log. In other types of hydraulic debarkers, a plurality of stationary or oscillating water jets are positioned around the circumference of a log, which transverses longitudinally, and the water jets separate the bark from the log.
While working well for some wood species and in some mill applications, hydraulic debarkers do not work well for species in which the bark is generally difficult to remove, and all hydraulic debarkers present operational disadvantages, in that very large volumes of water are used, and must be treated after use. Thus, the environmental concerns for water treatment are significant, and the cost for equipment and for operating water treatment facilities can be high. Additionally, bark removed by hydraulic debarkers is wet and soggy; therefore, being difficult to handle after removal.
A second general category of debarker currently used is referred to as mechanical debarkers. In one type of mechanical debarker, a plurality of cutting tools are positioned around the circumference of the log, and the tools are rotated around the log as the log travels therebetween, The tools strip the bark from the log. This type of debarker is inefficient for short logs, and is generally prone to mechanical failure. Additionally, mechanical debarkers work well only for log species in which the bark readily fractures. Stringy types of bark tend to separate from the log in large sheets and wrap around the cutting tools, thereby making handling difficult.
In another type of mechanical debarker, the logs are positioned on two support rolls, and a cutting or grinding roll is positioned above the log. As the log is rolled, the roll grinds the bark from the log.
A third general category of debarker is known as a drum debarker, in which a large drum having surface elements on the inside thereof for cutting bark, receives a plurality of logs at one time. The drum is rotated to tumble the logs, and the surface elements remove the bark from the log. Drum debarkers normally are designed with slotted openings for bark to fall from the drum.
Mechanical debarkers and drum debarkers do not work well on severely bowed logs when cutting tool contact along the log is restricted. In Northern climates, bark removal efficiency is reduced substantially when logs become frozen, regardless of the type of debarker being used. In some situations, it has been found necessary to subject logs to several consecutive debarking operations in order to achieve acceptable levels of bark removal.
Certain wood species present debarking difficulties. For example, eucalyptus, aspen, and other species at certain stages of log freshness or at certain times of the year actually debark too easily. The bark separates in large sheets, which clog debarking tools and generally are difficult to handle. In a drum debarker, bark from fresh eucalyptus will often accumulate in large balls that stay in the debarker and are removed therefrom only when the logs are removed. Apparatus must be provided to separate the bark pieces from the debarked logs, to ensure that the bark does not accompany the logs into subsequent log processing steps. In spite of these difficulties, drum debarkers are still most frequently used for eucalyptus, aspen and the like, in that the other types of debarkers are even less effective with these species than are the drum debarkers.
Handling bark removed from certain species is also difficult. The large bark sheets from eucalyptus can plug conveyors and will often intertwine in holding bins and adhere to each other, such that the entire bin of bark becomes a single mass. To alleviate this, separate bark handling process lines are often used, including chopping appratus for reducing the size of the bark pieces. Such apparatus is expensive to obtain and is subject to clogging and mechanical failure during operation.
In the past, complicated pretreating devices have been used with some log species in an attempt to facilitate bark removal and handling; however, most such devices have generally been prohibitively expensive and operationally ineffective.